Photolithography processes are used in semiconductor manufacture to pattern interconnect lines and other features for producing circuits on semiconductor wafer (substrate) surfaces. A conventional photolithography system generally includes components such as a light source, optical transmission and focusing elements, transparent reticles or photo masks, and process electronic controllers. The system is used to project a specific circuit or other feature image, defined by the mask reticle pattern, onto a semiconductor wafer coated with a light sensitive film (photoresist) coating. After image exposure, the film is then developed leaving the printed or other feature image of the circuit on the wafer.
Photo masks used today in semiconductor fabrication include conventional masks with a combination of circuit or other feature patterns formed by alternating transparent regions and opaque regions. Such photo masks typically consist of a substantially transparent base material such as quartz that allows the light to pass through certain regions with an opaque patterned layer having a desired circuit pattern formed thereon that prevents the light from passing through other certain regions. Materials such as chrome have been commonly used for forming the opaque layer and may typically have a thickness on the order of about 1,000 Angstrom. Other materials such as nickel and aluminum have also been used to form the patterned opaque layer on the surface of the photolithographic mask. Whereas conventional photo masks have a generally uniform thickness except for the very thin opaque chrome plated regions, phase shift masks or PSMs are photo masks in which certain regions of the transparent base material have different thicknesses. These latter “phase shift” regions cause a phase shift in the light traveling therethrough and minimize the effects of light diffraction through the photo mask for improved image resolution which may otherwise adversely affect formation of the intended pattern in the photoresist on the wafer. In some types of photo masks such as halftone phase shift masks, materials such as MoSiON has been used for the phase shifter material. In other instances, chromeless phase shift lithography (CPL) technology using chromeless masks have been used to the pattern the photoresist layer on the wafer.
As semiconductor fabrication technology advances to continually higher performing and smaller integrated circuit chips or dies, the accompanying circuits continue to become geometrically smaller and more densely packed on the chips. Accordingly, the pitch or spacing between circuit lines and other features formed on the wafer is concomitantly reduced.
Some problems associated with the shrinking circuit geometries found in the 90 nm and below semiconductor fabrication processes is contamination of the photo mask. As device features shrink, the minimum size threshold for surface contaminants that accumulate during use on the photo mask and which may adversely affect the photolithography process and proper patterning of the photoresist shrinks as well. Particulate contamination on the photo mask may cause defective images to be printed onto the semiconductor wafer which can render an entire chip unusable.
Periodic cleaning of photo masks is therefore necessary to extend mask life time by removing accumulated particulate from the surface of the masks to avoid defective printing and circuit formation problems. Some conventional approaches to cleaning photo masks has been the use of wet chemical cleaning processes using ammonia-based solutions such as SC1/APM (H2O2+NH4OH+H2O), DIH2/hydrogen water (H2O+H2+NH40H), and NGT (cluster H2O+ammonia gas). Photo mask cleaning may be enhanced by using these solutions in combination with acoustical Megasonic cleaning processes in both dip type or spin type (i.e. ultrasonic waves with frequencies typically higher than 700 KHz, such as 1 MHz and 3 MHz) known in the semiconductor industry. In conventional dip mask cleaning processes, the photo mask is placed into the chemical solution typically contained in a tank. Megasonic waves are then generated within the solution to improve particulate removal from the photo mask.
The foregoing chemical cleaning of photo masks, however, has drawbacks. These known process may seriously damage the pattern (opaque layer features) or sub-resolution assist features (SRAF). After chemical cleaning, an additional step of rigorously rinsing the photo masks with water must be also performed to remove residual ammonia which can otherwise lead to the formation and growth of precipitated chemical defects on mask's chrome pattern that may cause circuit printing and formation problems during photolithography. Such rinsing operations, however, are not always completely effective in removing all residual ammonia. In addition, the chemical cleaning processes raises environmental issues by generating waste chemical solutions that require costly proper disposal and is inconsistent with current “green” manufacturing process goals.
An improved non-chemical photo mask cleaning process is desired.